1. Field of the Invention
This invention relates to wear-resistant lubricant compositions suitable for use on surfaces that are in sliding contact with one another. The compositions are particularly useful for coating the surfaces of pistons and/or cylinders of engines using alcohol or other non-petroleum-based products as fuel.
2. Background Information
To increase the rotating velocity, compression ratio and mileage of diesel type engines and reduce their weight, there is an ever increasing demand for development of parts that are smaller and formed from light-weight metal alloys. This trend has resulted in an increased demand for improved resistance to wearing and seizure at the interfaces between parts such as pistons and cylinder walls that are in sliding contact with one another.
Due to a decrease in the supply of petroleum-based fuels, replacement of these fuels with alcohol-based fuels has been considered for diesel engines. The use of alcohol-based fuels has resulted in a high demand for increased resistance to wear and corrosion between stationary and sliding parts of diesel engines, such as at the interfaces between cylinder bores and piston skirt portions. Meeting this requirement has been the objective of much research.
For the sliding parts of diesel engines operating on non-petroleum-based fuels, when the stationary and moving parts, such as the cylinder bore and the skirt portion of a piston contain the same type of metal, seizure may occur due to sticking of metal parts of the same type. This is a disadvantage. In order to prevent sticking between metal parts of the same type, testing has been conducted with the objective of forming a resin coating layer on the surface of the piston skirt portion. In order to improve the sliding performance of the resin coating layer, various types of solid lubricants have been tried in combination with the resin.
For example, in the method disclosed in Japanese Kokai Patent Application No. Sho 51[1976]-97812, a type of resin composition for sliding type lubrication with improved durability of the resin coating layer is prepared by blending polyamide resin or silicone resin, or other heat-resistant resin as a binder with solid lubricants consisting of 10-75 wt % of graphite, 0.1-60 wt % of MoS.sub.2 (molybdenum disulfide), and 1-20 wt % of PTFE (polytetrafluoroethylene). The solid lubricants constitute up to 75 weight percent of the total composition.
Japanese Kokai Patent Application No. Sho 54[1979]-162014 discloses a type of resin composition for lubricating members such as pistons that are in sliding contact with another surface. The composition is a blend of polyamide resin and PTFE, and it is able to reduce wear and noise level of the pistons.
Japanese Kokai Patent Application No. Sho 62[1987]-63628 discloses a type of resin composition for lubricating members in sliding contact with one another. The compositions contain 85 weight percent of a polyamidimide resin, 10 weight percent of MoS.sub.2, boron nitride, graphite and other solid lubricants; and 5 wt % of PTFE.
Japanese Kokai Patent Application No. Hei 1[1989]-87851 discloses a type of resin composition for sliding type lubrication prepared by adding 25 to 125 parts by weight of PTFE to a 100 parts of a polyamide resin. The PTFE improves the wear resistance of the resin coating layer.
Japanese Kokai Patent Application No. Hei 4[1992]-175442 discloses a type of resin composition for sliding lubrication, such as between a piston and a cylinder bore. The composition contains 47 weight percent of a polyamidimide resin, 38 weight percent of MoS.sub.2, 9 weight percent of PTFE, and 6 wt % of carbon as graphite. This composition has an improved resistance to seizure.
Japanese Kokai Patent Application No. 87/34280 describes a corrosion-resistant bolt containing on its surface a layer of a finely divided lead powder bonded with three trivalent chromium compounds that is in turn covered with a layer of lubricant composition containing from 20 to 70 weight percent of PTFE and from 1 to 5 weight percent of an inorganic lubricant such as MoS.sub.2 or graphite.
Using any of the prior art lubricant compositions to reduce the coefficient of friction (.mu.) between members in sliding contact with one another, the proportion of the solid lubricants with respect to resin must be increased. Using this approach, as the proportion of the solid lubricants added in the resin composition is increased, the bonding strength between the solid lubricant particles and the resin binder decreases to the extent that the solid lubricant particles may drop off easily due to sliding of one member against the other. Consequently, the coefficient of friction tends to rise and the wear rate of the resin coating layer tends to increase.
Using PTFE as the solid lubricant, as the proportion of this ingredient is increased, the coefficient of friction tends to decrease. However, when the proportion of PTFE becomes the major ingredient of the coating, the trend reverses. As the coefficient of friction increases, the rate at which the resin coating layer wears increases and peeling may occur. This is due to degradation of the wetting property of the sliding surfaces.
Using MoS.sub.2 as the solid lubricants, as its relative concentration is increased, the coefficient of friction decreases, while the seizure resistance (load resistance) tends to increase. However, when the proportion of MoS.sub.2 exceeds a level determined at least in part by the particle size of the MoS.sub.2, there is a tendency for the surface roughness to increase and for the coefficient of friction to rise.
When carbon in the form of graphite is used as the solid lubricant, the seizure resistance, also referred to as load resistance, of the resin composition can be improved when MoS.sub.2 is also present. However, when the proportion of graphite becomes higher than a value determined by the composition of the lubricant, the strength of the resin coating layer is significantly decreased, and the wear rate of the resin coating layer tends to rise.
While PTFE, MoS.sub.2, and graphite are indispensable as solid lubricants, the present inventors have discovered that the relative concentrations of these ingredients as well as the total concentration of these ingredients relative to the resin matrix must be within specified ranges to achieve a low coefficient of friction and a low wear rate.
Resin layers formed from prior art resin-based lubricant compositions have poor initial conformability characteristics. Consequently, after a prescribed period of use as a lubricant in sliding contact environments, the coefficient of friction becomes larger than the initial coefficient of friction. This is related to the observed decrease in the wear resistance.